1. Field of the Invention
The present invention relates to a technology that is well adaptable to an electric storage device including plural positive electrodes, each having a different capacity.
2. Description of the Related Arts
An increased capacity or increased output is demanded for an electric storage device that is mounted in an electric vehicle, hybrid vehicle, or the like. Therefore, a lithium ion secondary battery, an electric double layer capacitor, and the like have been nominated as candidates for the electric storage device. However, the lithium ion secondary battery has a high capacity, but low output. The electric double layer capacitor has a high output, but low capacity.
In view of this, there has been proposed an electric storage device called a hybrid capacitor in which the electric storage mechanisms of the lithium ion secondary battery and the electric double layer capacitor are combined. The hybrid capacitor employs an activated carbon, which is used for the electric double layer capacitor, as a positive-electrode material. By this structure, charges are accumulated in the positive electrode of the hybrid capacitor by utilizing the electric double layer. On the other hand, the hybrid capacitor employs a carbon material, which is used for a lithium ion secondary battery, as a negative-electrode material. By this structure, lithium ions are doped into the carbon material of the negative electrode in the hybrid capacitor so as to accumulate charges. The application of the electric storage mechanism described above makes it possible to increase the capacity and output of the hybrid capacitor. However, a further improvement in the capacity and the output has been demanded in order to use the electric storage described above for a vehicle power source.
Methods for enhancing both of a capacity and output of an electric storage device include the one in which an internal resistance is decreased by coating an electrode mixture material to be thin, and the one in which a battery having a high capacity characteristic and a capacitor having a high output characteristic are connected in parallel. However, in the method in which the electrode mixture material is coated to be thin, the energy density of the electric storage device might be decreased. This method further entails a difficulty in assembling the device, which increases cost. In the method in which the battery and the capacitor are combined, cost might be increased due to a complicated control circuit.
In order to solve these problems, there has been proposed an electric storage device in which positive-electrode current collectors of a lithium ion secondary battery and an electric double layer capacitor are connected to each other, and negative-electrode current collectors of the lithium ion secondary battery and the electric double layer capacitor are connected to each other (e.g., see JP-A-2001-351688). Further, an electric storage device has been proposed in which a slurry including an activated carbon or the like and a slurry including a lithium cobaltate or the like are coated double on a current collector (e.g., see JP-A-2000-36325 and JP-A-2005-203131), or an electric storage device has been proposed in which a slurry having mixed therein an activated carbon and a lithium cobaltate is coated on a current collector (e.g., see International Publication WO2002/41420).
However, in the electric storage device disclosed in JP-A-2001-351688, it is difficult to cancel the deviation in the potential between the electrodes that are connected to each other. Therefore, overdischarge or overcharge might locally be produced in the positive electrode or the negative electrode. The overdischarge or overcharge described above causes the deterioration in durability of the electric storage device. The electric storage devices disclosed in JP-A-2000-36325, JP-A-2005-203131, and International Publication WO2002/41420 have a structure in which the activated carbon and the lithium cobaltate are mixed or a structure in which the activated carbon and the lithium cobaltate are coated in a double layer manner. In the electrode structures described above, it is difficult to sufficiently decrease the internal resistance. Further, the electric storage devices have a structure in which the lithium cobaltate is in contact with the activated carbon. Therefore, the affect caused by the deteriorated lithium cobaltate might affect the activated carbon, which deteriorates the durability of the electric storage device. Further, the electric storage devices disclosed in JP-A-2001-351688, JP-A-2000-36325, JP-A-2005-203131, and International Publication WO2002/41420 have a structure in which a large load is applied to the lithium cobaltate when the electric storage devices are charged or discharged with a high current. The electrode structure described above also causes the deterioration in durability of the electric storage device.